A comment on the nature of the composite operators in a QFT is in order. Let two fields \(\Phi _1\)  and \(\Phi _2\)  be inserted into the correlator at the same space-time point \(\mathbf {x}\) . Correlators typically diverge when the points of insertion of two or more fields coincide. Such singularities must be appropriately regularized and renormalized [1]}. For free field theories, `normal-ordering' is a convenient prescription to achieve this goal. Under this prescription, the two fields \(\Phi _1\)  and \(\Phi _2\)  are treated as a single composite operator inserted at \(\mathbf {x}\)  into the correlator. The composite operator is denoted as \(:\Phi _1\Phi _2:(\mathbf {x})\)  where \(::\)  is the normal-ordering symbol.
